I found this on line somewhere: I love the Steptronic but "best" is in the eye of the beholder. This says it way better than I can describe.
High-performance electric control for a fast response.
The new version of BMW six-speed automatic transmission allows a truly unique, active and sporting style of motoring. The sophisticated electronic control unit permanently monitors the position of the gas pedal, registering the intensity of the pressure exerted by the driver on the gas pedal and determining whether and to what extent the driver wishes to accelerate.
Retrieving data on the road speed of the car, engine speed and the steering angle, the control unit also detects current driving conditions at all
times, considering furthermore whether the car is driving uphill or downhill.
By taking all these criteria and many others into account, electronic
transmission management, together with its high-performance software,
is able to determine which gear is the most suitable under current driving
conditions to meet the driver’s particular demands.
The driver expresses these demands smoothly, easily and clearly through his foot on the accelerator, the transmission control unit automatically choosing the topmost gear as long as the driver is just cruising along with consistent pressure on the gas pedal. In this case the automatic transmission will never forget to shift up, keeping the car in its most economical gear at all times. But at the same time the automatic transmission is always ready for any change in driving conditions, responding instantaneously and precisely to new demands and requirements, regardless of whether the driver wishes to slowly build up extra speed or whether he obviously wishes to accelerate fast and dynamically.
Direct choice of the optimum gear.
Whether the transmission should shift back and, if so, to what degree, is
determined by the electronic control unit applying data provided on load and driving conditions as well as the rate at which the driver presses down the
gas pedal. So depending on the overall scenario, the transmission is able to shift down in the same brief instant either “just” one or several gears.
All that counts is how fast the driver wishes to accelerate. The harder he presses down the gas pedal, the more dynamically his car will accelerate
to the desired speed, choosing the gear required directly and without
any transitional phase.
This direct gear finding process, as it is called, gives the automatic transmission its unique precision, without any time-consuming search for the right gear, which the driver would always notice and experience as an unwanted
delay. Indeed, the transmission will determine and find the optimum gear ratio instantaneously while the driver is still pressing down the gas pedal,
making machine even faster than man.
A mere 100 milliseconds response time.
Responding at such unprecedented speed, BMW’s new six-speed automatic transmission sets new standards, the dynamic gearshift leaving behind
not only conventional automatic transmissions, but even the manual gearbox: The new six-speed automatic transmission completes every gearshift process faster than even the vast majority of sporting drivers experienced in shifting gears manually. This is borne out particularly clearly by the extreme demand made by BMW’s development engineers in defining a kick-down signal at a speed of 70 km/h or 50 mph in sixth gear. The requirement even in this case is to ensure maximum acceleration – and precisely that is what the new six-speed automatic transmission provides, responding in just 100 milliseconds to the driver’s commands. At the same time the automatic transmission sends
a positive force pulse to the engine, which immediately increases its speed from approximately 1,400 to more than 5,000 rpm while the transmission itself shifts back from sixth to second gear.
The time required for shifting gears is also shortened 50 per cent compared with a conventional transmission, again enabling the car to instantaneously switch over from a smooth cruising mode to fast and dynamic acceleration in less than a second, offering truly impressive shift-down performance
quite impossible with any kind of conventional automatic transmission.
The new six-speed automatic transmission therefore responds more quickly than the driver is even able to notice – all he feels is the spontaneous reaction to his commands and wish for dynamic acceleration. And all that remains
is a wonderful feeling of satisfaction, with the driver marvelling at the intuitive reaction of the new six-speed automatic transmission to his wish for dynamic performance
